The present invention relates to a method and an apparatus for detecting a pattern formed on a substrate by use of a laser beam of high illuminance and directivity (spatial coherence) as an irradiation light, or more in particular such a method and an apparatus for detecting an alignment pattern on a substrate in a TTL (Through The Lens) alignment of a reduction projection exposure system.
A conventional pattern detection apparatus is disclosed, for example, in JP-A-60-l363l2 (U.S. Pat. No. 4,668,089) as shown in FIG. 6. This conventional apparatus uses a laser beam high in illuminance and directivity (spatial coherence) as an irradiation light. When an object pattern is illuminated by laser beam, however, the reflected light has a component along the angle of diffraction from the direction of regular reflection representing the spatial frequency of the pattern of the object, and therefore the high frequency component is sharply cut by the spatial frequency corresponding to the outer periphery of the pupil (or the numerical aperture) of the detection optical system. Insertion of a spatial filter of such a sharp cutting characteristic generates a noise in the form of a vibration oscillation corresponding to the spatial frequency thus cut in the detected waveform, thereby posing the problem of the original pattern being deformed.